Conventionally, there is known an apparatus which detects the rotational speed of a brushed DC motor from the waveform of current flowing through the motor without need to use an encoder, a tachogenerator, or the like (refer to Patent Documents 1 and 2).
FIG. 9 shows the conventional pulse detection circuit which is described in Patent Document 1 and adapted to detect rotation pulses generated in association with rotation of a motor. The pulse detection circuit detects through current which flows to the motor from a current detection resistor R connected to the ground side of a motor driver, thereby detecting through current pulses generated in association with rotation of the motor.
The pulse detection circuit includes a differentiation capacitor C; a pulse amplification circuit, which includes an inverting amplifier circuit OA; and a comparison-and-judgment circuit. In the thus-configured pulse detection circuit, when the motor rotates forward or reversely, current pulses generated in association with rotation of the motor flow through the current detection resistor R. The pulses are input to the operational amplifier OA via the differentiation capacitor C and a resistor R1 and amplified therein. At this time, as shown at the upper part of FIG. 10, an output voltage proportional to time differential of a pulse voltage appearing across the current detection resistor R is output from the operational amplifier OA. A voltage divider composed of two resistors R2 and R3 halves a supply voltage Vcc and supplies to a noninverting input of the operational amplifier OA a voltage corresponding to ½ the supply voltage Vcc. Two-thirds (a voltage ratio between resistors R5 and R6) the supply voltage Vcc is input as a threshold voltage to an inverting input of a comparator Comp; thus, when the voltage of an input pulse signal is in excess of the threshold, pulses as shown at the lower part of FIG. 10 are output. In this manner, the illustrated circuit can yield the number of pulses proportional to the rotational speed of the motor through time differential of the pulse voltage appearing across the current detection resistor R. However, when motor current smoothly varies periodically, pulses cannot be detected from the motor current. Particularly, in a motor having four or more field poles, difficulty is encountered in reliably detecting pulses whose magnitude sufficiently exceeds a threshold value, from current flowing through the current detection resistor.
FIG. 11 is a sectional view showing a conventional motor disclosed in Patent Document 3. Field magnets are a first magnet of N pole, a second magnet of S pole, a third magnet of N pole, and a fourth magnet of S pole, wherein the first magnet of N pole and the second magnet of S pole are increased in magnetic flux density through elongation of their arcuate length, and the third magnet of N pole and the fourth magnet of S pole are reduced in magnetic flux density through reduction of their arcuate length. By means of using a pair of magnets of N pole and S pole having high magnetic flux density as a unit and a pair of magnets of N pole and S pole having low magnetic flux density as a unit, an attraction direction T is directed toward a position between the first magnet of N pole and the second magnet of S pole, which have high magnetic flux density. The magnets are disposed such that the magnetic centers of the arcuate surfaces of the magnets form equal spread angles about the center of a rotating shaft.
The illustrated field magnets are configured such that different sizes are imparted to the magnet surfaces which face the outer circumferential surface of an armature core, thereby establishing difference in magnetic force among the magnets. Accordingly, the armature core is attracted toward the magnets on one side, so that the shaft united to the armature core is attracted toward the one side. Therefore, even when a predetermined clearance is provided between the shaft and a bearing, the shaft can be prevented from having play, whereby noise can be reduced. However, merely through employment of difference in magnetic flux density among the magnets, there cannot be generated a sufficiently high pulse voltage for detection by such a pulse detection circuit as described in Patent Document 1 mentioned above.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2004-297861    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. H9-222433    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2007-104875